Wikipedia reports that there "is no formal process for selecting the Secretary General. Instead, the members of NATO traditionally reach a consensus on who should serve next. This procedure often takes place through informal diplomatic channels... The appointment is for 4 years, although it may be extended."
Control by Bilderberg

The main "informal diplomatic channel" would appear to be the Bilderberg meetings. Since the official narrative denies that this is anything more than a social event, albeit one with extreme secrecy, their control of who becomes NATO Secretary General is hard to prove. Nevertheless, it is interesting that at least since 1971, all permanent NATO Secretary General appointees have attended at least one Bilderberg meetings before being appointed to the job. Most of the others are known or suspected to have attended after their appointment:
Hastings Ismay, (1952-1957), Appointed before the first Bilderberg meeting. Possible Bilderberger.[1]
Paul-Henri Spaak, (1957-1961), Bilderberger (1963, possibly before)[2]
Dirk Stikker, (1961-1964), Bilderberger (1964, possibly before)[3]
Manlio Brosio, (1964-1971), Possible Bilderberger (1965-1967)[4][5][6]
Sergio Balanzino, (1994, 1995) Temporary appointee for 6 weeks, then again, for 9 weeks
Alessandro Minuto-Rizzo, (2003) Temporary appointee for 2 weeks

Office Holders on WikispooksName Image From To Deputy Description
Jens Stoltenberg 1 October 2014
Anders Fogh Rasmussen 1 August 2009 1 October 2014
Jaap de Hoop Scheffer 1 January 2004 1 August 2009
Alessandro Minuto-Rizzo 17 December 2003 1 January 2004
George Robertson 14 October 1999 17 December 2003
Javier Solana 5 December 1995 6 October 1999
Sergio Balanzino 20 October 1995 5 December 1995
Willy Claes 17 October 1994 20 October 1995 Resigned after the discovery of a bribe of over 50 million Belgian francs.
Sergio Balanzino 13 August 1994 17 October 1994
Manfred Wörner 1 July 1988 13 August 1994
Peter Carrington 25 June 1984 1 July 1988 A Bilderberg chairman.
Joseph Luns 1 October 1971 25 June 1984 Longest ever serving Secretary General of NATO
Manlio Brosio 1 August 1964 1 October 1971 Possibly the last Secretary General to be appointed without having attended a Bilderberg conference.
Dirk Stikker 21 April 1961 1 August 1964
Paul-Henri Spaak 16 May 1957 21 April 1961
Hastings Ismay 24 March 1952 16 May 1957 The first Secretary General of NATO, and quite possibly the only one who never attended a Bilderberg conference.